Process for recovering iron and aluminum compounds



Patented Sept. 22, 1942 mocass FOB. aEco ALUMINUM VERI'NG moN AND OMPOUNDS Louis F. Clark, Montebello, Calif.

No Drawing. Application December 7, 1938,

Serial No. 244,375

7 Claims.

This invention relates to the recovery of useful iron (or iron and aluminum) compounds from the sulphate solutions obtained from leaching ores, concentrates or calcines, or from pickling operations, i. e., in treating ferrous metals with sulphuric acid to remove oxide scale, etc. In usual practice such solutions are used until their activity falls by reason of the fixation of free acid and .because of the concentration of salts. Usedpickle solutions are then usually discarded, and frequently become a waste tions in which iron is the principal ingredient and from which a nearly complete removal of iron in one operation is sought. The processes described here are, moreover, analogous to those,

described in my co-pending application for Processes for recovering aluminum compounds," filed October 20, 1937, Serial No. 170,029.

One of the particular objects or the present;

invention is to treat the large volumes of waste pickle liquor in such a way that valuable iron salts can be separated therefrom without resorting to evaporation or crystallization.

Another object is to regenerate a portion of the acid which has been consumed in any such leaching operation, and to prepare a solution of low iron content containing this regenerated acid" which can be returned to the original operation. A further object is to produce an iron salt comparatively rich in iron, combinedwith some acid, and which will readily combine with sumcient additional acid to make a normal salt.

Still another object is to combine an ore leaching and solution purification process into one operation in an autoclave at high temperature,

as for example, in recovering manganese from low grade ores thereof.

The fundamental operation consists in treating solutions at high temperature and pressure generation of free acid. The reaction ends at an equilibrium according to the concentration of salts originally present, to the temperature reached, and to the amount of alkali salt present. Ferrous'iron' is not so hydrolysed and precipitated, hence, can be separated from ferric iron. Pickle liquor, carrying mainly-ferrous iron, must be oxidized either before or during the precipitation operation to accomplish iron recovery.

In the production ofprecipitated iron salt from waste pickle liquor, the solutions are to be heated in an autoclave, preferably by direct steam injection. It is desirable, therefore, that they be rich in iron. Anautoclave feed carrying to grams per liter metallic iron is desirable. -The solution should also be clarified, as by settling, so that no suspended insolublematter will be carried into the autoclave. If a large proportion of free acid exists in the solution, it should be largely consumed by reaction on soluble iron compounds such as siderite, mill scale, or espe- I Fe 2(S O4)a+I-I2O+trace HNOs This operation consumes some free acid, if

present. If no free-acid is available, the solution will become turbid with basic salt, but this is not important because we eventually precipitate most of the iron as basic salt anyway. The

oxidation by this operation should not be carried to completion; some ferrous iron should be left to be oxidized by the trace of nitrate when the solution is subsequently heated in the autoclave.

It should also be noted that the oxidation, wholly or in part, can be effected by the direct injection of oxygen into the solution while at high temperature and pressure. conditions this reaction is comparatively active,

even without the presence of any catalytic oxide.

of nitrogen, although a trace of such a catalyst may be used to advantage to save time. It is also useful to withdraw oxygen from the gas space above a solution and vreinject it again below the surface of the liquid so as to get dispersion precipitate basic salts, with simultaneous re- 55 of gas into e q Under these effective oxide slurry for this purpose.

spounds of precipitate. v,it not necessary to supply F6203 for all the free 75 The hydrolysis effected at the higher tempera- I tures may be represented as follows:

In this operation free acid is regenerated, and this, in conjunction with the solution condition and the temperature attained, determines the extent of the reaction, i. e., theproportionof the total iron which is precipitated as basic sulphate.

Assuming a solution in which nearly all the iron is ferric iron and which carries little if any free acid, there are yet other factors which can be adjusted so as to secure a precipitationof over 90% of the iron. These factors are the addition of an alkali salt, e. g., Na2S04 (or even -NaCl), and also the addition of soluble iron acid regenerated by hydrolysis in the autoclave, since in the presence of'alkali ion and at the higher temperature in the autoclave, the basic oxides, e. g., specially prepared limonite, to partthe objectives, the iron oxide used for this purpos'e should be low in insoluble material and.

finely pulverized, and made up into a slurry with water. This slurry should then, preferably, be injected into the autoclave after the solution therein has been brought to high temperature:

and pressure. Under these circumstances, the

, hydrolysis may be made nearly complete, as follows Z precipitate is formed in the presence of considerable free acid. Thus we can get over 90% precipitation of the iron originally present as F6504 by using a smaller quantity of both NazSO; and

F8203 than indicated above, and at the same time let the free acid in the mother liquor build up to 3-5% H2804. Thus the autoclave tail solution, after removal of precipitate, can be returned to the pickle operation and such free acid becomes available for useful work.

It will be noted, as above indicated, that the autoclave precipitate formed under these conditions is not simply a basic ferric sulphate, but is a double basic salt of alkali and iron and is analogous to the natural mineral natrojarcsite, NazS04.3Fez(0H)4.S04, although the actual precipitate formed in the autoclave may vary considerably from this type mineral formula, particularly in the ratio of alkali to iron.

This insoluble precipitate can be readily fi tered out of the mixture discharged from the autoclave and it can also be well washed on the filter. It has been found that such a double basic salt can be made soluble in dilute acids'by calcination under closely controlled furnace conditions, so that most of the water combined as hydroxyl group will be expelled, and yet prevent the loss of much S02 or S03. This conversion into the acid soluble form by calcination is very The iron compound used here to fix the regenerated free acid can also be mixed into the solution before heating in the autoclave, but it has been found that by allowing the hydrolysis to proceed as far as it will first, in the solution itself, then injecting the extra iron as a slurry, that the final precipitate is much more granular and easily filtered. Limonite dehydrated at 600 F. and then finely pulverized (as with a grinding mill in circuit with an air separator) makes an A. mechanical agitator in the autoclave is useful in helping to keep iron oxide in suspension until the reaction is complete.

A suitable iron compound for this purpose may also be prepared by precipation of a hydroxide sludge from the waste pickle liquor, or from washes obtained in leaching an ore, by soda ash. The precipitate is separated from the main mass of the liquid, and the slurry so formed directly injected into the autoclave, as described in my co-pending application Serial No. 170,029, for treatment of solutions of aluminum. Still other sources of iron oxide for this purpose are blast furnace fiuedust, pyrites cinder, calcine from the copper or zinc sulphide concentrates, or manganese ores prepared for leaching bya reducing According to the, above, 1.43 pounds soluble FB2O3+O.143 pound oxygen+0.85 pound Na2S04 for each one-pound Fe as FeS04 would tend to bring about complete precipitation of iron in 5.77 In practice, however, it

much more readily accomplished on this double basic salt than on basic ferric sulphate only,

which apparently begins to decompose (i. e., evolve S02 and S03) before it is completely dehydrated, thus merging from one insoluble form into another with less of the useful intermediate product', which is soluble in dilute acid, being obtained.

As an example of the operation of this process in pickling steel, 'let the operation be conducted was to discard a waste liquor at 1.17 sp. gr., carrying 15% FeS04 (5.5% Fe), and 1.70% free H2504. Every ton of such liquor would call for the addition of about 15 pounds oxygen, 150 pounds soluble ferric oxide and '75 pounds Na2S04, and enough steam to raise the temperature to 350 F. This treatment would produce about 500 pounds precipitate and a solution car rying about 3% H2804. Such a mixture as discharged from the autoclave is to be passed through a settling tank and the clarified solution returned to the pickling operation.

As further examples of the application of this process, consider the simultaneous leaching of copper or zinc sulphide concentrates and the precipitation of insoluble basic iron compound in an autoclave at high temperature' Here' the pulverized ore is suspended in a dilute solution of acid and iron sulphate, heated to about 350 F. and oxidized by injection of oxygen. (The presence of alkali ion and trace of nitrogen oxide is assumed but omitted for simplicity in the following equations.)

' 0r, leaching a mixture of pulverized sulphide mineral and oxidized ferruginous manganese or in the autoclave at high temperature.

0r, leaching the oxidized manganese one in the autoclave at high temperature with injection of sulphur dioxide.

It should also be noted that a sulphide concentrate may be partly or completely oxidized by roasting in a furnace before being subjected to the reaction in the autoclave, and also that the iron oxide used in the slurry for injection may itself be part of a double sulphide which has been calcined to make an acid soluble oxide.

Attention should also be drawn to the fact that solutions of aluminum can also be treated by the methods herein described, whereby with suitable additions of soluble iron oxide and alkali, followed by heating in the autoclave to about 350 F., both the iron and aluminum can be nearly completelyprecipitated as a complex basic iron aluminum alkali sulphate. Conversely, solutions of iron may be treated with additions of soluble alumina to produce the same result.

Having thus described the general character of my processes for the treatment of sulphate solutions of iron, and given examples of its practice, it will be evident to those familiar with this art that various modifications may be made or special applications devised without departing from the process as defined in ,the following claims.

I make, therefore, the following claims:

1. The method of treating sulphate solutions of assorted metals including iron and aluminum sulphate 'to selectively precipitate a double basic sulphate of trivalent metals of the group consisting of aluminum and ferric iron, andalkali metal, which consists in adjusting the proportion of alkali metal so that the solution contains approximately one mol of alkali metal sulphate for each mol of said trivalent metal sulphate in the solution, adding approximately two mols of the oxide of said trivalent metal for each mol of said trivalent metal sulphate in the solution, and heating the mixture in an autoclave at a temperature above 300 F.

2. The method of treating sulphate solutions of assorted metals including iron and aluminum sulphate to selectively precipitate a double basic sulphate of trivalent metals of the group consisting of aluminum and ferric iron, and alkali metal, which consists in adjusting the proportion of alkali metal so that the solution contains approximately one mol of alkali metal sulphate for each mol of said trivalent metal sulphate in the solution, adding approximately two mols ferric oxide for each mol of said trivalent metal sulphate in the solution, and heating the mixture in an autoclave at a temperature above 300 F.

3. The method of treating sulphate solutions of assorted metals including ferrous sulphate and aluminum sulphate to selectively precipitate a double basic sulphate of trivalent metals of the group consisting of aluminum and ferric iron, and alkali metal, which consists in adjusting the proportions of alkali metal so that the solution contains approximately two mols alkali metal sulphate for each six mols ferrous sulphate in the solution, adding approximately three mols ferric oxide for each six mols ferrous sulphate in the solution, heating the mixture in an autoclave at a temperature above 300 F., and oxidizing all iron to the ferric state. c

4 The method of treating sulphate solution of assorted metals including ferric sulphate to selectively precipitate a double basic sulphate of ferric iron and alkali metal similar to the jarosite minerals which consists in adjusting the proportions of alkali metal so that the solution contains approximately one mol alkali metal sulphate for each mol ferric sulphate in the solution, adding approximately two mols ferric oxide for each mol ferric sulphate in the solution, and heating the mixture in an. autoclave at a temperature above 300" F.

5. The method of treating sulphate solutions of assorted metals including ferrous sulphate to selectively precipitate a double basic sulphate of ferric iron and alkali metal similar to the jarosite minerals which consists in adjusting the proportions of alkali metal so that the solution contains approximately two mols alkali metal sulphate for each six mols ferrous sulphate in the solution, adding approximately three mols ferric oxide for each six mols ferrous sulphate in the solution, heating the mixture in an autoclave at a temperature above 300 F., and oxidizing all iron to the ferric state.

6. The method of treating sulphate solutions of iron including ferrous sulphate to precipitate a double basic sulphate of ferric iron and alkali metal similar to the jarosite minerals which consists in adding approximately three mols of ferric oxide, and two mols of' alkali metal sulphate for each six mols of ferrous sulphate in the solution, oxidizing the dissolved iron and heating the mixture in an autoclave to a temperature above-300 F.

.7. The method of treating sulphate solutions of iron including ferric sulphate to precipitate a double basic sulphate of ferric iron and alkali metal similar to the jarosite minerals which consists in adding approximately two mols ferric oxide and one mol alkali metal sulphate for each mol of ferric sulphate in solution and heating the mixture in an autoclave at a temperature above 300 F.

LOUIS F. CLARK. I 

